This invention relates to a process for preparing crystalline ammonium dihydrogen phosphate (NH.sub.4 H.sub.2 PO.sub.4) which is high in whiteness and in the columnar form, and more particularly to the same process for preparing ammonium dihydrogen phosphate from wet process phoshoric acid.
In accordance with a conventional process for preparing ammonium dihydrogen phosphate from wet process phosphoric acid, a formed crystalline ammonium dihydrogen phosphate is in the form of needle crystal which is colored by the effect of various impurities. These impurities include organic compounds and inorganic metallic ions.
The organic compounds may be removed to a considerable extent by means, for example, of oxidation or reduction treatment, or active carbon treatment. However, the metallic ions are considerably difficult to be removed even by such a treatment, because they are relatively liable to enter the crystal of ammonium dihydrogen phosphate and tend to be concentrated by some treating manners. Additionally, the crystal form of ammonium dihydrogen phosphate becomes needle-like by the effect of such impurities.
In this regard, it has also been proposed to repeat the recrystallization of ammonium dihydrogen phosphate crystal, by which the impurities in the crystal of the ammonium dihydrogen phosphate can be removed to some extent. However, such a proposition encounters the following disadvantages from point of view of industrial manufacturing: not only process is complicated, but also the yield of ammonium dihydrogen phosphate is liable to decrease, contributing to economical disadvantage. Additionally, even by this proposition, the crystal form of ammonium dihydrogen phosphate can not improved to a desired extent.
Ammonium dihydrogen phosphate is usually crystallized in the needle-like form in case where the phosphate is prepared by the reaction of wet process phosphoric acid and ammonia. Such needle-like crystal is disadvantageous because it is not effective for filtration, and bulk density is considerably low increasing storing and wrapping costs. Furthermore, the needle-like crystal is relatively liable to be blocked in a container.
Now, such crystalline ammonium dihydrogen phosphate is used as the starting materials for various phosphates, baking powder, boiler compounds, decoloring agent for wool, powdered fire extinguishing agents and food additives. Of these, the use as the fire extinguishing agent is recently increasing, the powder of which agent is required to be uniform and in the form of fine particles. It is to be noted that as the powdered fire extinguishing agent, columnar or granulated crystal is desirable because the needle-like crystal is not suitable to be injected from a fire extinguisher.